The present invention relates generally to a zoom lens, and more particularly to a zoom lens system having a low telephoto ratio.
So far, two-group zoom lens systems have been used primarily for zoom lenses having a relatively low magnification, and three-group zoom lens systems have been adopted mainly for zoom lenses having a higher magnification. These zoom lens systems are operated in various zooming modes; however, this results in an increase in the number of lens groups involved. To reduce the number of lens groups and lens elements, the aperture ratios of the systems at their telephoto ends are reduced depending on their operating modes, while full use is made of aspherical surfaces.
For zoom lenses used on compact cameras, there is a zoom lens system disclosed in JP-B 8-3580 published under the name of the applicant. The zoom lens system set forth in JP-B 8-3580 comprises, in order from its object side, a first lens group having positive refracting power, a second lens group having positive refracting power and a third lens group having negative refractive power. For zooming from the wide-angle end to the telephoto end of the zoom lens system, each lens group moves toward the object side. This zoom lens system becomes shortest at the wide-angle end and longest at the telephoto end.
This zoom lens system is characterized in that the amount of zooming movement of the third lens group is larger than those of the rest, thereby achieving a higher zoom ratio. The publication teaches that focusing is effected by the movement of the second lens group, thereby achieving the most stable image-formation capabilities.
For four-group zoom lenses used on compact cameras, on the other hand, there is a zoom lens system disclosed in JP-B 8-30783 published under the name of the applicant, which comprises, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having negative refracting power. From the wide-angle end to the telephoto end of the zoom lens system, each lens group moves toward the object side.
This zoom lens system is characterized in that the zoom ratio is allocated to each lens group by the movement of each lens group, thereby achieving a high magnification. For zooming from the wide-angle end to the telephoto end, the second lens group is allowed to move toward the object side, so that size reductions are achievable at the wide-angle end.
Some proposals have been made to divide each lens group into subgroups for zooming movement (JP-A's 7-27979, 8-29688, 8-110471 and 9-159918).
To reduce the size of lens systems as mentioned above, many proposals have so far been made of taking full advantage of aspherical surfaces having great ability to correct for aberrations, thereby reducing the number of lenses forming one lens group. Many proposals have also been made of using a radial type gradient index lenses.
Most of actual zoom lens systems for compact cameras have a mechanism for accommodating them in camera bodies irrespective of their types. To this end, a lens collapsible mount mechanism is provided in a space defined in a lens position at the wide-angle end where the zoom lens system becomes shortest to move the zoom lens system therein. It is thus possible to reduce the size of the camera during lens collapsing.
In taking actual shots, however, the zoom lens system must be used in the form of the original optical system, and so a lens mount (for holding the lens system) projects from the camera body. Especially in the case of a zoom lens system having a high zoom ratio, the amount of movement of lenses in association with zooming becomes large at the telephoto end. This means that the overall length of the lens system, i.e., the lens mount increases largely. For this reason, the center of gravity shifts more largely at the telephoto end than during lens collapsing or at the wide-angle end, making the balance of the camera worse and rendering the camera difficult to handle.